1. Field of the Invention
The present invention relates to a trench MOSFET structure with a guard ring and the method for manufacturing the same, and more particularly to a structure of a trench MOSFET which solves low breakdown voltage in contacted trench gate area and the method for manufacturing the same.
2. The Prior Arts
In the structure of a trench Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) or vertical transistor, the gate of the transistor is formed in a trench on top of a substrate and the source/drain regions are formed on both sides of the gate. This type of vertical transistor allows high current to pass through and channel to be turned on/off at a low voltage.
Referring to FIG. 1, a cross-sectional diagram of the structure of a trench MOSFET is shown. An N-type doping epitaxial region 105 is provided on a N+ substrate 100. A plurality of trenches 106 are formed on the N-type doping epitaxial region 105 that has lower doping concentration than the substrate 100. The surface of trenches 106 which is covered a gate oxide layer 110 thereon are filled with a doped polysilicon layer to form a plurality of trenched gates 115. A plurality of P-type doping regions 120 are formed on both sides of the trenched gates 115. A plurality of N+ source regions 125 are formed in the P-type body regions 120. A metal layer 160 is deposited on the top of the MOSFET structure for formation of source metal 160b, and gate metal field plate metal 160a (gate metal 160a also services as field plate metal here) of the MOSFET. An insulating layer 130 is formed on top surface of the epitaxial layer and the trenched gates 115 for insulating. Metal contact plugs 137 are formed in the P-type body regions 120 for connecting source metal 160b, and in a wide trench of the said trenched contact gates 116 for connecting gate metal 160a (The wide trenched contact gate 116 allows metal contact having more tolerance into the doped polysilicon in the wide trench without shorting to the P-type body regions 120 resulted from misalignment). A plurality of P+ heavily-doped regions 121 are formed at the bottom of the metal contact plug 137. The MOSFET structure of the prior arts also has a P-type guard ring 170 which is formed aside the P-type body regions 120 underneath the field plate metal 160a of the MOSFET to increase breakdown voltage in termination. However, the structure in FIG. 1 has low breakdown voltage occurring on trench bottom of the trenched contact gate 116 as result of wider trench which has deeper trench depth than the trench depth in active area. The trench depth is deeper when the trench width is wider because more open area allows more etching gas goes into trench during dry etching silicon process. When reverse bias between drain and gate/source increases, avalanche will first occur on the trench bottom of the trenched contact gate 116 because it has deeper trench gate.
The present invention provides a new structure of trench MOSFET structure with a guard ring wrapped around the contacted trench gate which improves the lack of the prior art.